The present invention concerns methods of making, or fabricating, integrated circuits, particularly methods of forming silver interconnects.
Integrated circuits, the key components in thousands of electronic and computer products, are interconnected networks of electrical components fabricated on a common foundation, or substrate. Fabricators typically use various techniques, such as layering, doping, masking, and etching, to build thousands and even millions of microscopic resistors, transistors, and other electrical components on a silicon substrate, known as a wafer. The components are then "wired," or interconnected, together to define a specific electric circuit, such as a computer memory or microprocessor.
Interconnecting millions of microscopic components typically entails covering the components with an insulative layer, digging small holes in the insulative layer to expose portions of the components underneath, and digging trenches from each hole to one or more other holes in the layer. Then, through metallization, the holes and trenches are filled with aluminum (or an aluminum alloy) to form aluminum interconnects, or wires, between the components.
To fill the trenches and holes, fabricators cover the entire insulative layer with a thin layer, or film, of aluminum, and then selectively dissolve, or etch, away the aluminum that lies outside the holes and trenches. The selective etching requires the use of photolithography, a photographic-patterning technique, to form an etch-resistant mask, which protects the aluminum-filled holes and trenches from the etchant. The resulting aluminum wires, intended to be flush, or coplanar, with the surface of the underlying insulative layer, are typically about one micron thick, or about 100 times thinner than a human hair.
These conventional interconnection techniques suffer from at least three significant shortcomings. First, because of the difficulty of using photolithography to form high-precision masks on bumpy, uneven surfaces, conventional techniques require digging trenches to ensure that the deposited aluminum wires are flush, or coplanar, with the surface of the underlying insulation. However, digging these trenches is a time-consuming step which ultimately increases the cost of manufacturing integrated circuits.
Secondly, conventional techniques produce wires of aluminum, which not only has a higher electrical resistance, but also a lower electromigration resistance than other metals, such as silver. High electrical resistance wastes power, and low electromigration resistance means that, at certain electric current levels, the aluminum readily diffuses, or migrates, into neighboring structures, eventually thinning or breaking the wires and thus undermining reliability of integrated circuits.
Moreover, although silver has a 40-percent lower electrical resistivity and at least 100-percent higher electromigration resistance than aluminum, conventional interconnection techniques are impractical for making silver interconnects. In particular, silver, a noble metal, is immune to most etchants. In fact, attempts to selectively etch a layer of silver covered with an etch-resistant mask usually dissolve the mask, not the silver. Thus, conventional etch-based techniques are wholly inadequate to form silver interconnects.
Thirdly, in addition to being time-consuming because of the trench-digging step and ineffective with more desirable metals such as silver, conventional techniques place aluminum wires in relatively high-capacitance insulators, typically solid silicon oxide. High capacitance slows the response of integrated circuits to electrical signals, a great disadvantage in computers and other systems including the integrated circuits.
Accordingly, there is not only a need for new interconnection methods that eliminate the trench-digging step, but also methods that yield less-resistive, less-capacitive, and more-reliable silver-based interconnects for faster and more-efficient integrated circuits.